This invention relates to side plates for cooling grate conveyors such as those described in U.S. Pat. Nos. 3,735,858 and 5,121,831, incorporated herein by reference thereto.
It is now established steel making practice to crush and concentrate iron ore, blend the ore with a fluxing material and form the blend into pellets. The pellets are sintered into a firm consistency at high temperature and cooled on traveling grates. The pellets are confined to the grate surfaces by traveling side plates, sometimes referred to as cooler side plates, attached to the grate conveyor chains which move the traveling grate. The bottom face of the side plates travel on steel support rails generally having a BHN hardness of about 450. Some fine grit or ore dust, which is highly abrasive, passes through the system along with the pellets and falls on the rails causing fairly rapid wear of the bottom of the plate. Additionally, the grate and side plates may operate at moderate to high temperatures and undergo continual temperature fluctuations. There is also some chemical attack by hot gases and also probably by the fluxing substances.
To try and cope with these multiple problems the grates and side plates have typically been cast from ACI type HH heat resistant alloy, which contains 24 to 28% Cr, 11 to 14% Ni, 0.2 to 0.5% C, up to about 1% each of Mn and Si, and the balance essentially Fe plus minor impurities. There is presently a trend toward using ACI type HI alloy, which contains 26 to 30% Cr, and 14 to 18% Ni, but is otherwise formulated like alloy HH. It is thought that alloy HI resists hot corrosion better and outlasts alloy HH. However, both alloys show undesirably rapid wear on the bottom edge of the side plates, resulting in a need to frequently shut down the system in order to replace the worn parts. Another method of overcoming this problem has been to bolt shoes of cast cobalt base alloys, which typically contain about 0.6 to 2% C, about 30% Cr plus 4.5% or more W and oftentimes about 1.5% Mo, to plates made from alloy HH or alloy HI. But, since cobalt is a relatively scarce element these cobalt-base alloy shoes are quite expensive. Also, they tend to crack and fall off during use, and the hand fitting and bolting required increases the expense of this method considerably. Thus, it remains highly desirable to find a more practical but far less costly method of overcoming the side plate wear problem.
One possible solution to these problems which was considered was to cast the side plate around some type of wear bar, composed of a material that would have high hot hardness and ability to resist hot abrasion. In order for such a wear bar to function successfully it would have to remain firmly in place and not crack, fragment, spall or melt in service. While many possible candidate materials such as ceramic oxides, borides, nitrides, silicides and cemented carbides, high carbon cobalt-chromium base alloys, high-carbon nickel-chromium alloys, high-carbon nickel-chromium-iron alloys, high-boron nickel-base alloys, would appear to be useful because of their hardness, it was found that such materials do not possess many other properties required. For example, the ceramic oxides were too brittle and susceptible to thermal and mechanical cracking and falling out of place. Further, ceramic borides, nitrides and silicides are generally far more costly to produce and have properties quite similar to the oxides in terms of early mechanical failure. Cemented tungsten, molybdenum and titanium carbides, which are commercially available, found to be far too costly for further consideration. Cast wear bars of cobalt-chromium-tungsten-molybdenum wear-resistant alloys tend to crack when the side plates are cast around them. Also, there is no practical method of separating these cobalt alloys from the surrounding side plate castings when scrap or defective castings are produced or for purposes of recycling used castings. Additionally, the cobalt-base alloy remains excessively expensive.
Fesler, U.S. Pat. No. 5,106,577 and Culling, U.S. Pat. No. 5,202,087 disclose high-carbon austenitic steels to meet similar hot corrosion and abrasion conditions in cement cooler grates. Both of these alloys displayed outstanding hot abrasion and corrosion resistance but had a high. tendency to crack in service due to their relatively poor resistance to thermal shock and fatigue. Also, inserts cast from those alloys tend to crack and/or come loose during the process of casting the side plates around the wear bar inserts. Cast high-carbon martensitic alloys such as the 15% Cr-3% Mo type, proved to be extremely brittle in this application. These alloys contain large amounts of hard, brittle carbides in a very brittle martensitic matrix.